1. Field of the Invention
This invention relates to a system for monitoring and detecting an occurrence of abnormality of an idle speed control system for an automobile engine.
2. Description of Related Art
Typically, in diagnostic techniques for diagnosing malfunction or abnormalities of an idle speed control system for engines, diagnosis is carried out without judging the idle speed control system functionally normal immediately after the engine has attained an idle speed of rotation within a target range. This is based on consideration that the idle speed control system possibly encounters malfunctions or functional abnormalities even after a lapse of a certain time since the engine has attained a target idle speed of rotation. One of the techniques, which is known from, for example, Japanese Unexamined Patent Publication No. 7-77093, is available as an onboard malfunction diagnostic system to efficiently accumulate reliable diagnostic data.
The diagnostic technique described in Japanese Unexamined Patent Publication No. 7-77093 comprises a first timer to count a time passing from a point of time that the idle speed control system enters into a feedback control mode, a speed monitoring means to detect whether the engine attains a rotational speed within a specified range, and a second timer to count a time passing from a point of time that the engine attains a rotational speed within the specified range, and judges the idle speed control system normal in function on the basis that the time counted by the first timer is less than a specified time, and the time counted by the second timer reaches another specified time.
With the prior art diagnostic technique, in cases where, although the idle speed control system has a abnormality, the engine instantaneously attains a rotational speed within a specified target range, it is prevented to make, at the instantaneous attainment of a target speed of rotation, a wrong judgement that the idle speed control system is normal in operation. However, because the specific range of rotational speed for abnormality judgment is invariable, in cases where, although the idle speed control system is functionally normal, it is unavoidable to make such pretended judgment that the idle speed control system is functionally abnormal when the engine causes a deviant rise in rotational speed exceeding the specified range of rotational speed.